At present there are various polymerization processes for preparing toner particles. Such processes have become of great interest in recent years due to the desire to overcome problems associated with toners obtained from pulverized processes. Indeed, the polymerization process allows for a better control of the shape and size distribution of the particles. Polymerization processes are thus known in the art. For example, it is known that toners can be prepared by mini emulsion polymerization/aggregation, by emulsion polymerization/aggregation or by suspension polymerization. These processes do require a surfactant in various degree of amount in order to complete the polymerization successfully.
Indeed, the surfactant plays an important role in polymerization. For example, the surfactant helps in the stabilization of the emulsion/mini emulsion/suspension polymerization compositions. Also, the surfactant helps in the formation of a stable colloidal dispersion and has a strong influence on monomer conversion; viscosity; particle size and their distribution. However, it has been noted that the presence of the surfactant creates problems in the final toner material. For example, surfactant on the toner particles absorbs the moisture contributing to a high relative humidity, which imparts poor adhesion between the toner and the substrate, low tribo charge, dielectric loss, aging and poor toner flow.
In order to circumvent these problems and obtain quality toner particles, basically two options are available. A first option is to remove the surfactant after polymerization and a second option is to avoid the use of any surfactant in the polymerization process. The second option, i.e., surfactant-free polymerization is the preferred option due to the fact that surfactant removal is tedious and resource consuming, which leads to a cost ineffective process.
The emulsion polymerization presents an advantage over mini emulsion polymerization in that it is easily scalable. Indeed, emulsion polymerization does not require any specific homogenization device to generate the emulsion droplets. Moreover, emulsion polymerization differs from suspension polymerization in that it allows for the production of particles with smaller sizes and distribution. Also, emulsion polymerization has an advantage over suspension polymerization in that it allows to simultaneously attain both high molecular weights and high reaction rates. Indeed, soap-free emulsion polymerization offers some advantages over conventional emulsion polymerization such as ease of purification, due to the absence of surfactant. Another advantage of surfactant-free emulsion polymerization lies in benignity of the process and in that it is free from surface active agent.
Jitka Solc (EP0209 879A2) describes a method of producing pigment-containing polymer dispersions by conventional emulsion polymerization. The pigment is encapsulated in styrene and butyl acrylate polymer by emulsion polymerization. The resulting encapsulated particles are employed in toners.
Takahiro Takasaki (U.S. Pat. No. 6,544,706 B1) describes an disclosure related to a production process of polymerized toner of a core-shell structure by suspension polymerization. The combinations of styrene with butyl acrylate (i.e., n-butyl acrylate), and styrene with 2-ethylhexyl acrylate are used as core. Styrene and methyl methacrylate, are used either singly or in combination as shell.
Tiarks et al. (2001) describe the encapsulation of carbon black by a mini emulsion polymerization method. The incorporation of carbon black is achieved by mixing a surfactant stabilized carbon black dispersion and a monomer mini emulsion.
Akasaki et. al., (U.S. Pat. No. 5,219,943) discloses a process for producing monodispersed fine particles of a vinyl polymer, comprising polymerizing monomers containing at least one vinyl monomer, and a methacrylic ester in the presence of a surfactant, a persulfate polymerization initiator, and a divalent metal as an electrolyte. In this process, surfactant is used to stabilize the emulsion during emulsion polymerization.
Nagai et al. (1999) describe the synthesis of carbon black dispersions of high stability, performed by copolymerizing a polymerizable surfactant (1-nonylphenyloxy-2-deca(oxyethylene)-3-alloxypropane ammonium sulfate SE-10N) with acrylonitrile in the presence of carbon black and water. The stability of the dispersion depends mainly on the conversion of the surfmer and the amount of the surfmer adsorbed on carbon black.
Casado et al. (2007) describe polymer encapsulation of water-dispersible, surface-sulfonated carbon black using surfactant-free emulsion polymerization of butyl acrylate, methyl methacrylate and allyl methacrylate. The investigation focuses on the effect of carbon black and the initiator on the conversion of monomer. 40 wt. % of initiator with respect to monomer is employed to get 100% monomer conversion.
Qui et al. (2005) describe a surfactant-free synthesis of styrene and sodium styrene sulfonate (NaSS). The effects of NaSS concentration and the order of addition of the reactants are examined in detail. The results show that the particle size decreases with an increase in the styrene sulfonate concentration. The polydispersity index can be reduced by mixing NaSS with styrene homogeneously before adding the initiator, but this leads to a slightly larger mean particle size.
Adelnia et al. (2014) describe a soap-free emulsion polymerization (SFEP) of methyl methacrylate, butyl acrylate in water/methanol media with sodium salts of four different acidic comonomers, namely, styrene sulfonic acid (NaSS), 2-acrylamide-2-methyl-1-propane sulfonic acid (NaAmps), acrylic acid (NaAA), and itaconic acid (Na2ita). It is found that the introduction of methanol as co-solvent (35 wt %) to the medium greatly decreases the amount of water-soluble polyelectrolyte in the cases NaAA and Na2ita, while it does not make any difference for NaSS and NaAmps. The addition of sulfonic-based co-monomers (NaSS and NaAmps), first decreased particle size and then led to predomination of solution polymerization over surfactant free emulsion polymerization (SFEP). On the contrary, the incorporation of carboxylic-based comonomers (NaAA and Na2ita) led to an increase in particle size.
Brijmohan et al. (2005) describe a sodium styrene sulfonate (NaSS) as an emulsifying co-monomer for surfactant-free emulsion polymerization of styrene and divinyl benzene (DVB). The NaSS oligomers stabilize the polymer particles by forming copolymer with styrene and DVB. The particles are stable in water yielding particle size in the range of 80-90 nm. The particles have broad size distribution which can be attributed to dual nucleation mechanism due to the presence of large amounts of surface active agents. The particle size increases with increasing amounts of DVB in the feed.
Chieh-Min Cheng et. al. (U.S. Pat. No. 6,458,501) depicts an disclosure related to a surfactant-free emulsion polymerization process and to a method for preparing emulsion aggregation toners wherein the latex is formed by surfactant-free emulsion polymerization of styrene/n-butyl acrylate/2-carboxyethyl acrylate (βCEA) copolymer. The preparation of a toner that comprises blending a colorant, preferably a colorant dispersion, more preferably containing a pigment, such as carbon black, phthalocyanine, quinacridone or rhodamine B type, with a latex polymer prepared by surfactant-free polymerization has been illustrated. In this process, surfactant is added to the colorant dispersion. In particular, using the surfactant-free latexes in emulsion aggregation toner generally enables at least 85% surfactant reduction since the bulk of the surfactant in typical toners comes from the latex rather than from the colorant dispersion.
Accordingly, as outlined above, it is known in the art that carbon black encapsulation can be done either by a process involving water dispersible surface modification of carbon black or by using a surfactant in the polymerization process. Also, it is know that the preparation of a polymerized toner can be carried out by suspension/emulsion/mini emulsion polymerization using a surfactant. It is also known that latex can be prepared by a surfactant-free process; however surfactant is added to the colorant dispersion or aggregation composition, the amount of surfactant used at the various stages of the process may be different but their presence is required to facilitate the polymerization for the preparation of the toner.
There is a need for a more cost effective and environmentally friendly process for the preparation of carbon black material.